Fiber reinforced composites (FRC) are useful, for example, as substitutes for metal materials. A fiber reinforced plastic wire can have strength comparable to a steel wire but only a fraction of the weight thereof. Fiber reinforced plastic wires can also be made more aesthetically pleasing to the eyes than steel wires do. They are therefore good replacements of metal wires used, for instance, in orthodontic treatments.
FRC wires can be produced by pultrusion. In a typical pultrusion process, a continuous reinforcement is first impregnated with curable resin and then pulled through a rigid die having a tunnel with a desired cross-section. The resulting long wire has a fiber inner core and a composite cover layer. The composite is cured in the die so that the wire can retain its cross-sectional shape. When cured, a soft material is hardened as chemical bonds are formed between atoms and/or molecules, which may occur for example under radiation. For example, monomers may be cured to produce polymers. After curing in the die, the resulting wire will retain a cross-section similar in size and shape to that of the die tunnel.
If desirable, the wire is subject to further processing such as longitudinal shaping and further curing. Post-pultrusion processing is commonly referred to as “beta-staging.” Under known approaches, to shape a wire longitudinally, the composite is only partially cured in the die, separated from the die, longitudinally shaped, and then fully cured.
An example FRC pultrusion process is described in U.S. Pat. No. 5,869,178, issued on Feb. 9, 1999 to Kusy et al. (“Kusy”), the content of which is incorporated herein by reference.
However, the known approaches to producing pultruded FRC have certain drawbacks. For example, under these known approaches, fiber distribution in the resulting composite wire is often uneven when the fiber content is in a certain percentage range. The manufacturing process can be complicated as two curing steps are required if the composite wire is to be shaped longitudinally. It is also difficult to produce very thin wires as it is difficult to insert resins into a very small opening of the die. Further, pulling a fiber through a very thin tunnel may induce high stress in the fiber. A highly stressed fiber is easy to break, either during or after the pultrusion process.
Thus, there is a need for improved fiber reinforced composites and improved methods of forming fiber reinforced composites.